Moisture-curable adhesives and sealants are formulated to react with moisture to form a cured polymer layer with high strength and adhesive properties. Moisture is provided in the ambient air or may be adsorbed on the surface or absorbed within the porosity of many substrate materials.
Moisture-curable resins include cyanoacrylates, silicones, and polyurethanes. The cyanoacrylates operate as adhesives, and the silicone and polyurethane resins are used commonly as sealants, caulking, and gasketing, but they can also be employed as adhesives.
Silicone-type sealing materials typically include a polymer having a silicon-containing group that has a hydroxyl or hydrolyzable group bound to a silicon atom and can be crosslinked to form siloxane bonds. They have excellent weathering resistance and heat resistance.
Previous methods of preparing these sealing compositions include copolymerizing a crosslinkable silyl-containing (meth)acrylic monomer and another vinyl monomer. However in this method, silyl groups are randomly introduced into molecular chains, instead of at the ends of the chains. Also, a (meth)acrylic monomer has been polymerized in the presence of a crosslinkable silyl-containing mercaptan, a crosslinkable silyl-containing disulfide and a crosslinkable silyl-containing radical polymerization initiator. Acrylic monomer has been polymerized in the presence of a crosslinkable silyl-containing hydrosilane compound or a tetrahalosilane. It can be difficult by these methods, however, to introduce the crosslinkable group into the polymer at both termini with sureness. Thus, an insufficient gel fraction and insufficient curability results. Further, it can be difficult to control cross-linking density and to obtain a narrow molecular weight distribution. A method for preparing silicone-type moisture-curable sealant materials would be desirable.
Furthermore, the characteristics of the sealant compositions such as mechanical properties and viscosity upon melting depend upon the ratio between the number of block segments of the block copolymer and the molecular weight of the blocks. It would therefore be desirable to control the microstructure of the block copolymer.
In other areas, polymer molecules with long-chain side branches and more than one junction point have useful rheological properties. The idealized molecule, called a “pom-pom,” has a single backbone, sometimes referred to as a crossbar, with multiple branches emerging from each end. The branches can become entangled with surrounding molecules, and the backbone can be stretched in an extensional flow, producing strain hardening. However, the shear rheology of these branched polymers can be shear thinning, as the backbone stretches only temporarily and eventually collapses as the molecule is aligned, producing strain softening.
The unique viscoelastic response of certain pom-pom polymer molecules in tension give them the ability to resist debondings and make them useful in adhesive applications. Branched polymers can also be used in elastomer formulations to improve green strength. Branched polymers can be prepared via anionic coupling reactions, but rigorous purification of solvents and monomers and careful fractionation of the polymer products is required. Also, when the methods are employed, it can be difficult to control and select the advantageous molecular weight of the branches and backbone for adhesive or elastomer applications.